This invention relates to a moisture sensor comprising a moisture-sensitive film having conductive particles dispersed in a hygroscopic polymer.
Among known moisture sensors for detecting humidity and moisture condensation, those comprising a moisture-sensitive film having conductive particles dispersed in a hygroscopic polymer are disclosed in JP-A 58-99740, 59-170755, 60-250241, 62-21052 and 6-249813. These moisture sensors are designed such that at a low humidity, conductive particles are in contact with each other to form conductive paths, providing a low resistance, but at a high humidity, the hygroscopic polymer absorbs moisture and swells thereby to interrupt conductive paths, changing to a high resistance state.
Another example of the moisture sensors using hygroscopic polymers is a moisture sensor comprising a polymer electrolyte having quaternary ammonium base whereby a change of its ionic conductivity is utilized (see JP-A 7-318526). This moisture sensor detects a humidity in an environment under observation by utilizing the phenomenon that the degree of dissociation of counter ions to the quaternary ammonium base varies with the level of moisture in the environment to change the ionic current flow upon application of voltage. However, the moisture sensor utilizing the change of ionic conductivity requires AC for measurement and thus needs a complex circuit. Additionally, the sensor performance is unstable in that at a high humidity above relative humidity (RH) 90%, especially under a moisture condensing atmosphere, the sensor is less resistant to water because the polymer electrolyte can be partially dissolved out. Another problem causing unstable performance is the hysteresis phenomenon that the sensor produces different output values at the same humidity depending on whether the humidity is increasing or decreasing.
By contrast, the aforementioned moisture sensors having conductive particles dispersed in a hygroscopic polymer have the advantage that the measurement circuit is simplified because of possible DC measurement. The sensors of this type, however, can detect only a range from RH 90% to moisture condensation, the detectable range being limited to a high humidity region. The hygroscopic polymers used in the prior art are hydroxymethacrylate and polyvinyl alcohol, which abruptly increase their water pickup when the surrounding humidity is beyond RH 90%. The sensor fails to produce a sufficient resistance value at a humidity below RH 90% and is thus low sensitive in the low humidity region.
JP-B 61-33374 discloses a moisture-sensitive resistor having conductive particles dispersed in an alcohol-soluble copolymerized polyamide resin. This resistor starts to increase its resistance at about RH 60%. Once exposed to a high humidity atmosphere, the resistor cannot resume the original resistance change and shows a resistance about twice the initial resistance value. The polyamide resin starts moisture absorption at a relatively low humidity, but the moisture pickup itself is small. In order to increase the change of resistance of the moisture sensor relative to a humidity change, the loading of conductive particles must be reduced. This leads to the drawback that the sensor shows high and unstable resistance values under dry conditions. The sensor of JP-B 61-33374 showed a resistance of about 5 kxcexa9 and a resistance change of about 3.5 places under dry conditions, which are still insufficient.
None of the moisture sensors comprising a moisture-sensitive film having conductive particles dispersed in a hygroscopic polymer are successful in achieving stable detection of a humidity below RH 90%. In the application where moisture sensors are incorporated in air conditioners and humidifiers, needed are moisture sensors which can detect a humidity not only in the high humidity region above RH 90%, but also in a low humidity region, especially in the humidity region of RH 60% to 90%.
Therefore, an object of the invention is to provide a moisture sensor which allows for DC-measurement, can detect a humidity in a region below RH 90%, especially in the humidity region of RH 60% to 100%, and ensures high performance stability against repetitive operations. Another object of the invention is to provide a moisture sensor which experiences little deterioration of performance even when kept in a high temperature, high humidity atmosphere.
The invention is directed to a moisture sensor comprising a pair of electrodes and a moisture-sensitive film disposed therebetween having conductive particles dispersed in a hygroscopic polymer.
In a first embodiment, the hygroscopic polymer is a polymer comprising a polyether amine having at least two amino groups and an epoxy compound having at least two epoxy groups and an ether bond. Preferably, the polyether amine has a polyether skeleton consisting of propylene oxide, ethylene oxide or a mixture thereof; the polyether amine is terminated with two or three primary amino groups; the polyether amine has a molecular weight of 100 to 5,000. Also preferably, the epoxy compound is of the following formula (1): 
wherein A is a di- to tetra-valent aliphatic saturated hydrocarbon group, aliphatic ether group or heterocyclic group, and k is an integer of 2 to 4. Further preferably the hygroscopic polymer has an amine equivalent and an epoxy equivalent which are in a ratio of from 1:1 to 1:4.
In one preferred embodiment, the hygroscopic polymer is a polymer comprising the polyether amine, the epoxy compound, and a water-soluble nylon, or a mixture of the polymer between the polyether amine and the epoxy compound and a water-soluble nylon. The preferred water-soluble nylon has an ether structure. The water-soluble nylon is preferably comprised of units of the following formula (7) or units of the following formula (8) or both: 
wherein R1 is an alkyl group of 2 to 4 carbon atoms. An appropriate content of the water-soluble nylon is 0.1 to 50% by weight based on the weight of the polyether amine and the epoxy compound combined.
In a second embodiment, the hygroscopic polymer comprises a polyether ester amide. The polyether ester amide is preferably of the following formula (2): 
wherein R is an alkyl group of 2 to 12 carbon atoms, m is an integer of 6 to 12, n is an integer of 1 to 4, p is an integer of 1 to 70, q is an integer of 5 to 200, and r is an integer of at least 3, and especially m is equal to 11 and n is equal to 2.
In one preferred embodiment, the hygroscopic polymer is a polymer comprising the polyether ester amide, a water-soluble nylon, and an epoxy compound having at least two epoxy groups, or a mixture of the polyether ester amide and a water-soluble nylon. The preferred features of the water-soluble nylon are as described above. An appropriate content of the water-soluble nylon is 0.1 to 50% by weight based on the weight of the polyether ester amide.
Typically, the conductive particles are carbon black particles having a specific surface area of 30 to 300 m2/g.
The moisture sensor can detect a humidity in the range of RH 60% to RH 100%.